This disclosure covers the field of emulsion chemistry. In particular, it relates to distinct solution based polymerized latex compositions that are initially shelf stable emulsions prior to being used as coatings and/or paints. More specifically these latex compositions are kept shelf stable in the presence of a specific amount of ammonium hydroxide to maintain high pH in order to avoid premature interaction (pre-gelling) between latex particles leading to settling, and both inter and/or intraparticle crosslinking of the latex binders. These solutions are ammonia (NH3) rich (using ammonium hydroxide) and thus highly basic; therefore, when the NH3 evaporates quickly, the pH of the solutions are reduced as they are applied to surfaces. This process serves as a trigger for controlled crosslinking of the latex (binder) as it interacts with the polyfunctional amines of the present disclosure during application and drying. The pursuit of fast drying aqueous traffic paints requires there be strong and effective interactions between the latex binder and water-soluble polyfunctional amine crosslinkers, to ensure fast hardening at proper high build (in a single coat thick application) translating into corresponding water resistance.
In an increasing number of industries, aqueous coating compositions continue to replace traditional organic solvent-based coating compositions. Paints, inks, sealants, and adhesives, for example, previously formulated with organic solvents are now formulated as aqueous compositions. This reduces potentially harmful exposure to volatile organic compounds (VOC's) commonly found in solvent-based compositions. Migration from organic solvent-based to aqueous compositions allows for health and safety benefits, however, the aqueous coating compositions must meet or exceed the performance standards expected from solvent-based compositions. The need to meet or exceed the organic solvent based performance standards places a premium on the characteristics and properties of waterborne polymer compositions used in aqueous coating compositions.
The latex industry and specifically the latex-based traffic paint products have historically held a long established goal of developing effective “one-pack” (proper high build-in a single coat thick application)—or single step crosslinking systems. The ideal system allows for film formation prior to substantial crosslinking as the latex is applied to surfaces. The nature of this coating technology requires that it is stable when being stored and fast drying only when being applied. The structural make-up of these aqueous systems must be unreactive in the wet state, but very capable of ionic bonding (in ambient conditions) in the dry state; referred to hereinafter as latent crosslinking. The result of latent crosslinking would be a good film-forming latex with excellent hardness that is very resistant to water wash-off.
Much published art regarding various “one-pack” chemistries exists, including those based on epoxies (specifically glycidyl methacrylate), silanes, isocyanates, and carbonyls (including acetoacetoxy ethyl methacrylate, and acetoacetoxyethyl methacrylate-AAMA). Most of these publications and/or granted patents have demonstrated the presence of crosslinking by showing improved solvent resistance.
In order to increase the potlife (or shelf stability) of compositions containing acetoacetate and amine groups it has been known to block the amine groups of the polyamine with a ketone or aldehyde to form corresponding ketimine or aldimine compounds prior to mixing with an acetoacetate-functional polymer. Examples of such non-aqueous compositions are disclosed in U.S. Pat. No. 4,772,680. Even though improved stability may be achieved by specific aromatic aldimines, volatile by-products are still formed and the compositions have no application in waterborne coatings and are restricted to coatings using organic solvents as the carrier.
WO 95/09209 describes a crosslinkable coating composition comprising an aqueous film forming dispersion of addition polymer comprising acetoacetate functional groups and an essentially non-volatile polyamine having at least two primary amine groups and wherein the mole ratio of acetoacetate to primary amine groups is between 1:4 to 40:1.
EP 555,774 and WO 96/16998 describe the use of carboxylated acetoacetoxyethyl methacrylate latexes mixed with multifunctional amines (such as diethylene triamine) for a shelf-stable, one-component system. In EP 555,774, the system is stabilized by using vinyl acid polymerized with AAEM and the latex is “neutralized” with a polyamine. The patent teaches that the carboxyl groups should be 70 to 96 mol percent relative to the acetoacetoxy groups. WO 96/16998 similarly describes a polymerization process with the vinyl acid and AAEM being polymerized in the first stage.
EP 744,450 describes aqueous coating compositions containing acetoacetate functional polymers with a weight-averaged molecular weight of 100,000 or greater and which contain acetoacetate functional groups and acidic functional groups, and multifunctional amine.
EP 778,317 describes an aqueous self-crosslinkable polymeric dispersion comprising a polymeric component (a relatively hydrophobic polymer having a Hansch number >1.5, at least 5% of a carbonyl functional group capable of reacting with a nitrogen moiety, and at least 1% of a non-acidic functional group having hydrogen-bondable moieties); and a crosslinking agent comprising a nitrogen-containing compound having at least two nitrogen functional groups capable of reacting with a carbonyl functional moiety. Again it is reported that no gelation has taken place after ten days at 60° C.
U.S. Pat. No. 5,498,659 discloses a single-package aqueous polymeric formulation consisting essentially of an evaporable aqueous carrier, at least one polymeric ingredient having acid-functional pendant moieties able to form stable enamine structures, a non-polymeric polyfunctional amine having at least two amine functional moieties, and an effective amount of base for inhibiting gelation. It is stated in the patent that at least some of the crosslinking of the composition may take place in the liquid phase, possibly within one to four hours of adding the non-polymeric polyfunctional amine. It is postulated that addition of base to the reactor contents competes with the amine-functional moieties vis-à-vis the acetoacetoxy-type functional moieties, thereby reducing the degree of crosslinking and/or enhancing the colloidal stability of the polymer dispersion which forms when the crosslinking reaction takes place.
Geurink, et al., in their publication “Analytical Aspects and Film Properties of Two-Pack Acetoacetate Functional Latexes”, Progress in Organic Coatings 27 (1996) 73-78, report that crosslinking of acetoacetate functional latexes with polyamine compounds is very fast, and that this crosslinking is hardly hindered by existing enamines. It is further stated that there are very strong indications that crosslinking takes place rapidly in the wet state, in or at the surface of the particles just after mixing of the components. They conclude that as a result of crosslinking in the particles, the film forming process is hampered.
In the publications described above, the usable pot life of the latex formulations is demonstrated by lack of sedimentation. It is quite possible, however, that crosslinking is taking place within each particle, without causing the latex to coagulate or gel (e.g. loss of colloidal stability). This type of intra-particle crosslinking (before drying) limits the ability of the latex to form a film upon drying. This in turn reduces the film integrity and performance of the polymer. Therefore, a need still exists for truly latent crosslinking systems—those in which intraparticle crosslinking is inhibited until after film formation. In particular, a need exists for one-pack, latent crosslinking systems which are useful in a wide range of latex applications that are simple and cost efficient. These would include decorative and protective coatings, adhesives, non-woven binders, textiles, paper coatings, traffic markings, inks, etc. In each case, the advantage would be a soft, ductile polymer that converts to a harder, more resistant latex film after drying.
In general, the following acronyms are used throughout the body this specification and provide information regarding chemical compounds and structures as follows;
ADS=ammonium dodecyl sulfate
APS=ammonium persulfate
BA=butyl acrylate
DMAPA=N,N-dimethyl amino propyl amine.
MMA=methyl methacrylate
MAA=methacrylic acid
PEI=polyethyleneimine
SDS=sodium dodecyl sulfate
tBHP=t-butyl hydroperoxide
TGIC=Triglycidyl isocyanurate